Many operations require the depth or height of a particular feature to be accurately determined. One such operation is blasting that occurs, for example, in a mining or construction context.
In downhole blasting operations drilled holes are filled with a charge or explosives which are then detonated. To ensure a safe and effective blast operation, it is important to be able to measure the depth of the shaft with a relatively high degree of accuracy. The depth of the shaft is required to calculate the amount of explosive required and the likely effects of the blast. In addition, where downhole blasting is being used, for example, to expose a coal seam it is important that holes of a known and correct depth are drilled.
One simple way to measure the depth of such a shaft is to secure a weight to the end of a length of measuring tape and allow the weight to fall into the shaft. The depth of the hole is then read off the tape at the surface. Such a method, however, presents a number of disadvantages. For example, during use the tape rubs against the ground and eventually, after repeated use, the measurement markings are obscured. The tape (a relatively expensive component) must then be discarded. Further, the tape itself can be bulky and difficult to handle especially when relatively deep holes are being measured (˜20 m and above).
As an alternative, a weight can be tied to an unmarked line and this can be dropped down the hole. The length of line played out can be measured to determine the depth of the hole. This approach, however, also raises a number of difficulties. For example, the line needs to be protected from the abrasive dust and dirt or else it will degrade and snap very quickly. A reliable way of measuring the line played out must also be available, one that will not be fouled by dirt and that provides an easy way to be operated in the field. Measuring drums are often used with fine gears for manual reading and these rapidly foul and fail. Electronic measures frequently use photo-optic sensors which also have limited tolerance of dust and dirt.
In addition, the phenomenon of ‘birds nesting’ or line overrun is an issue. This occurs when the weight hits the bottom of the hole but the reel holding the line continues to spin due to it rotary momentum. This causes the line to play out in excess of the requirement since the weight is already stationary at the bottom of the hole. Not only can this result in inaccurate measurement, but also often results in a large tangle of line that can be impossible to untangle.
For many shafts it is not possible to have a user positioned directly over the shaft while taking a measurement. This may be due to the debris extracted from the shaft during drilling being piled up around the edges of the shaft and/or surface instability around the mouth of the shaft. Accordingly, the user will often need to stand a distance from the mouth of the shaft and cast the line in, which can require a relatively high degree of manual coordination. In the action of casting, inaccuracies in the measurement are introduced (due to not knowing the distance from the user to the mouth of the shaft) and potential for knotting or tangling the line is increased.
Some holes can have water or mud at the bottom and it is often important to determine the depth of mud or water at the bottom of the hole in addition to the overall depth of the hole. To do this the operator typically ‘bounces’ the tape in his or her hand in an effort to feel the transition between the drag of the water or mud and the air. This requires a relatively heavy and large weight in order for the operator to be able to feel a difference between the tape/weight hanging in the air and the tape/weight hanging in water. The stiffness and weight of the tape itself can add to the difficulty in detecting the surface of the water. Although detecting the surface of the water is difficult, it is the established current process in the open cut mining industry. Quite often the water level measurement that is recorded is merely a best guess rather than an accurate measure due to the difficulties described above.
Most, if not all, handling/measurement is done manually and the bulk of the tape means that it is often quite heavy and difficult to handle/manipulate. In some cases, however, the tape may be connected to a winch or similar to allow the tape to be retracted from the hole with ease. This can, however, also lead to problems. During the retraction foreign matter collected by the tape from the shaft and surrounds (dust, clay, mud etc) is pulled into the workings of the winch by the tape, which can in turn lead to operational failure. The bulk of a winch also creates operational difficulties as it accelerates fatigue and can be a trip hazard.
While some of the above problems can, to an extent, be mitigated by an experienced user, the requirement to have such an experienced user to determine the depth of all blast holes itself can be problematic. If only one or two people on the working party have the experience to consistently and accurately determine the depth of the shafts this can cause significant delays in an operation.
More complex automated measurement methods have been trailed. These include the use of lasers and echo location to determine the depth of holes. While not technically impossible, these methods have very specific operational requirements that mean that the user would have to be very skilled and conscientious in the operation of the device. Both lasers and sonar type devices require the beam (optic or sonic) to be very carefully aligned with the axis of the hole and they are very sensitive to outcrops or debris in the hole which may falsely signify the bottom of the hole. As such when a reading is obtained, there is sometimes quite significant uncertainty as to the accuracy of the reading which has to be assessed against other information such as the intended depth of the hole.
Furthermore, both optical and sonic systems have great difficulty in differentiating between mud and water, and as cannot easily determine the true depth of the hole or the depth of mud or water.
It would be desirable to provide an apparatus for measuring depth which overcomes or ameliorates one or more of the above problems. In the alternative, it would be desirable to provide consumers with a useful.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.